Electric utility power generating systems generally comprise an alternating current electric power generator driven by a turbine. While some systems employ water turbines, most systems utilize steam turbines in which a controlled steam flow through the turbine regulates the rotational velocity of a driven turbine shaft. The steam flow is controlled, in response to electric power demands placed on the generator, such that the frequency of the alternating current produced by the generator is maintained at a constant value regardless of variations in electric power demands placed on the generator. The steam flow in turn is controlled by various flow control valves such as throttling valves and steam bypass valves.
The present invention relates to an improvement in the flow control valves of a steam turbine and especially to a throttling valve. In the past, these valves had a valve plug riding in a bonnet cylinder with the valve plug having one or more pressure seal ring grooves therein. A pressure seal ring can maintain the pressure drop across the ring which results in the pressure seal ring contacting the valve plug. In service, it has been found that the pressure seal ring wears a step in the contact surface along the edge of pressure seal ring groove of the valve plug. The resultant step worn along the edge of the groove can result in a wedging of the pressure seal ring between the valve plug and the bonnet cylinder bore and the resultant inability of the valve to close as required for turbine overspeed protection. The aim of the present invention is to prevent this wedging of the pressure seal ring between the valve plug and the bonnet cylinder bore as the pressure seal wears into the grove wall of the valve plug.
Typical throttling valves and steam bypass valves can be seen in prior U.S. Patents to Heymann for "Noise Suppressing Throttle Valve", No. 3,857,542, and in the Brown et al. U.S. Pat. No. 3,602,261, for "Steam Turbine Control Valve Structure", and in the Dawawala et al. U.S. Pat. No. 4,679,769, for "Steam Turbine Control Valve for Cyclical Duty". This latter patent shows an overall configuration of a one-piece bonnet control valve. These patents are all assigned to Westinghouse Electric Corporation.
In current practice, the valve plug is guided in the bonnet cylinder bore by clearances adjacent the upper and lower ends of the valve plug and by the pressure seal rings. A ball joint type connection joins a valve stem to the valve plug to float within the cylinder bore. Resistance to sideways or orbiting motion is limited to friction at the ball joint connection and the friction dampening between the pressure seal rings and the associated valve plug grooves.
More recently, there has been developed a throttling valve with a pressure seal ring positioned in a seal ring groove about a valve plug in which the groove includes a relief groove formed in one wall of the groove adjacent a bottom surface of the groove. The relief grove defines a reduced width wall surface in one wall of the groove which is narrower than the width of the seal ring. As a consequence, the narrow bearing surface wears evenly at least to the depth of the relief groove to reduce opportunity for wedging or binding of the seal ring. A description of this arrangement is set forth in U.S. Pat. No. (S.N. 298,443) assigned to Westinghouse Electric Corporation. While the invention set forth in that patent does provide one method of mitigating the effects of seal ring groove wear and reduces the potential for seal ring wedging, it does not provide positive alignment of the valve.